Abnormal combustion, more commonly known as knock, is a limiting factor on power generation in an internal combustion engine. The name knock refers to the noise that is transmitted from the collision of multiple flame fronts and the increased cylinder pressure that causes the pistons, connecting rods and bearings to resonate. A knock sensor detects engine knock and sends the voltage signal to an engine control unit (ECU). The ECU uses the knock sensor signal to control timing of the engine. Engine knock generally occurs within a specific frequency range and the knock sensor is located on the engine block, cylinder head or intake manifold and is able to detect that frequency range.
Detecting the presence of a capacitive sensor, such as the knock sensor, at the end of a capacitive cable, through an engine control unit (ECU) that is protected by an electromagnetic compatibility (EMC) capacitor is a long existing problem. This is because the knock sensor has high impedance and does not generate a very strong output signal, which means that the sensitivity of the ECU is critical. In addition, due to legislation relating to emission controls in engines there is a legal requirement to know if the knock sensor is working or not. Knock is a relatively quiet noise which can be easily confused with other engine noises and the determination of whether the knock sensor is working or not is made particularly difficult by this. Another problem that exists is that the capacitance of the knock sensor is relatively low and is often hard to measure due to the values of the EMC capacitor and the capacitance in the cabling.
A number of different methods have been proposed to overcome the problem of detecting the presence of the capacitive sensor. One relates to diagnostics using threshold detection as is described in for example EP 0720007 (Bosch). In this method the normal output of the sensor is characterised with respect to engine speed, and output higher or lower than expected is taken to indicate a fault with the knock sensor. The issue with this is that the point of the knock detection system is to minimise knock, which therefore means the normal condition is nascent or no knock and hence low output from the sensor. Since the ECU input is high impedance it tends to be subject to significant pickup of electrical noise. Thus low noise from a connector sensor and electrical pickup by the wires when the sensor is disconnected can be indistinguishable.
A second method of overcoming the problem that has been disclosed is a diagnostic using self resonance as is described in U.S. Pat. No. 5,421,191 a (Chrysler). In this method the sensor is excited with pulses matching the sensor's predetermined resonance and the amplitude of these is measured after excitation is removed. This method requires knowledge of the resonant frequency of the sensor, wiring and load combination as installed, which can vary considerably. If the driving frequency is wrong then no resonance will be induced.
A third method of overcoming the problem that has been disclosed is a diagnostic using capacitor oscillation, as is described in EP 0704706 (Marelli). In this scheme the sensor is used as part of the oscillator. If the sensor is connected then a particular frequency should be presumed. This scheme suffers from tolerance issues. The sensor is around 800 pF and the connector connecting it might be 1 m to 5 m of coaxial cable at 300 pF/m. The EMC protection capacitors are typically between 470 pF and 1000 pF. As these capacitor are usually low tolerance it becomes difficult to identify whether the sensor itself is connected due to the greater amount of low tolerance capacitance in the connection to the sensor.
One object of the present invention is to overcome at least some of the problems associated with the prior art. In addition, a further object is to provide a diagnostic of capacitive sensing by charge sharing.